This invention relates to a pump; more particularly, to a liquid helium pump having a superconductive electromagnetic drive which includes a stationary superconductive energizing solenoid connected to the pump housing and a superconductive element which is movable by means of the field generated by the energizing solenoid and which is fixedly attached to the pumping member proper of the pump.
Pumps of the above-outlined type are needed for driving liquid or supercritical helium in a closed circuit or, in general, for displacing liquid helium.
Particularly in the field of fusion technology, the use and operation of coreless fusion magnets are indispensable. The coils of the fusion magnets--which generate magnetic fields of a flux density in the order of a few Tesla--are constituted by hollow superconductors, which may have a length up to 1,000 m and which are to be cooled with helium which is in a single phase. The displacement of the helium is effected at a pressure above 2.4 bar and at a temperature of approximately 4.2 K. The pressure difference between inlet and outlet is in the order of magnitude of 1 bar. In particular modes of application, such as in installations for testing superconductive hollow conductors as a function of the parameters of the flowing helium, such as pressure and flow quantity, there is further required a constant capacity and a constant pressure difference during the pump cycle. These requirements are not met by known piston pumps driven by an eccentric.
In an article entitled "A Reciprocating Liquid Helium Pump Used for Forced Flow of Supercritical Helium" by G. Krafft et al (Cryogenics, February 1978), there is disclosed a piston pump for driving liquid helium. The pump drive is arranged externally of the cryostat and therefore the piston rod has to pass through the cryostat wall. Such an arrangement requires a vacuumtight and heliumtight seal and is involved with substantial expense. These difficulties are circumvented in another known liquid helium pump described in an article entitled "Heat Transfer by the Circulation of Supercritical Helium" by H. H. Kolm et al (Advances in Cryogenic Engineering, Volume 11, Plenum Press, New York, 1965). In this arrangement, the ferromagnetic piston is, with bellows, accommodated in a pump housing and a solenoid for driving the piston is arranged externally of the pump housing. The pump housing which necessarily is made of a non-magnetic material, on the one hand, increases the gap between the solenoid and the piston and, on the other hand, it does not eliminate undesired effects of foreign (external) magnetic fields. It is another disadvantage of this type of pump that it has only a small output of approximately 6.4 cm.sup.3 /s.
In an article entitled "An Electrically Pumped Liquid Helium Transfer System" by B. Darrel et al (Advances in Cryogenic Engineering, Volume 11, Plenum Press, New York, 1965), there is disclosed a liquid helium pump wherein the superconductive driving coil is a disc coil mounted at an end of a bellows-equipped superconductive piston received in the pump housing. The piston is moved by the driving coil by attraction and repulsion. The forces exerted on the piston change substantially during each stroke, because as the distance between the piston and the driving coil increases, the forces exerted on the piston drop sharply. Further, in this type of pump too, strong foreign magnetic fields may significantly interfere with the pumping operation. The output of this pump too, is low; it is only approximately 7 cm.sup.3 /s.
In the Handbuch fur Hochfrequenz- und Elektrotechniker, (Manual for the High Frequency Technician and Electrotechnician), Volume 1 3rd Edition (published by Verlag fur Radio- Foto- Kinotechnik Berlin, 1952), on pages 438 and 439 there is described a drive system for an electromagnetic loudspeaker. The drive system comprises a pot-shaped electromagnet having a central core which carries an energizing solenoid and the free end of which forms, with a disc-shaped pole shoe, an annular gap into which swings the voice coil for moving the loudspeaker diaphragm when the voice coil is excited with a voltage in the sound frequency range.